The present invention relates to a phase shift photomask used as an original drawing for a reduction projection aligner.
Hitherto, formation of fine-line patterns for solid-state devices, e.g., VLSI, has been carried out mainly by a reduction projection exposure method. In this method, light is applied to a photomask, which has a magnified pattern drawn thereon, by using a projection optical system so that a reduced image of the pattern is formed on a substrate coated with a resist by light transmitted through the photomask, thereby transferring the pattern.
As the density of patterns becomes higher, the pattern separation decreases, and it has become close to the wavelength of the light source in recent years, resulting in poor resolution and contrast of the projected image. To improve the resolution and contrast of the projected image, a phase shift photomask has been proposed, and a technique wherein a second opening pattern is provided in a peripheral region adjacent to a first opening pattern to further improve the resolution has been proposed as is disclosed in Japanese Patent Application Laid-Open (KOKAI) No. 62-67514 (1987).
The conventional phase shift photomask suffers, however, from the following problem: In the actual semiconductor manufacturing process, a fine-line pattern must be transferred to the whole area of the wafer surface having steps of 1 .mu.m to 2 .mu.m. With the conventional phase shift photomask, however, when a pattern is simultaneously formed at different focus positions, there is considerable variation in the dimension of the transferred pattern.